Hydrocarbons and NOx emissions are a direct result of the combustion process in an internal combustion engine. To reduce such harmful emissions, catalytic converters are employed to reduce their toxicity. For gasoline engines, “three-way catalysts” are used to reduce nitrogen oxides to nitrogen and oxygen (2NOx→xO2+N2), oxidize carbon monoxide to carbon dioxide (2CO+O2→2CO2); and oxidize hydrocarbons to carbon dioxide and water: CxHy+nO2→xCO2+mH2O. In the case of compression ignition or “Diesel” engines, the most commonly employed catalytic converter is the diesel oxidation catalyst. This catalyst employs excess O2 in the exhaust gas stream to oxidize carbon monoxide to carbon dioxide and hydrocarbons to water and carbon dioxide. These converters virtually eliminate the typical odors associated with diesel engines, and reduce visible particulates, however they are not effective in reducing NOx due to excess oxygen in the exhaust gas stream.
Another problem prevalent with diesel engines is the generation of particulates (soot). This is reduced through what is commonly referred to as a soot trap or diesel particulate filter (DPF). The catalytic converter itself is unable to affect elemental carbon in the exhaust stream. The DPF is either installed downstream of the catalytic converter, or incorporated within the catalytic converter itself. A clogged DPF can create undesired backpressure on the exhaust stream and thereby reduce engine performance. To alleviate this problem, the DPF can undergo a regeneration cycle when diesel fuel is injected via a dosing valve directly into the exhaust stream and the soot is burned off. The injection of diesel fuel can be stopped after the regeneration cycle is complete.
NOx emissions in the exhaust from a diesel engine can be reduced by employing a Selective Catalytic Reduction Catalyst (SCR) in the presence of a reducing agent such as ammonia (NH3). Existing technologies utilize SCR and NOx traps or NOx absorbers. The ammonia is typically stored on board a vehicle either in pure form, either as a liquid or gas, or in a bound form that is split hydrolytically to release the ammonia into the system.
An aqueous solution of urea is commonly used as a reducing agent. The urea is stored in a reducing tank coupled to the system. A dosing valve is disposed on the exhaust carrying structure upstream of the catalytic converter to meter the delivery of a selected quantity of urea into the exhaust stream. When the urea is introduced into the high temperature exhaust, it is converted to a gaseous phase and the ammonia is released to facilitate reduction of NOx. In lieu of ammonia, diesel fuel from the vehicle's fuel supply can be used as the reducing agent. In this expedient, a quantity of diesel fuel is administered directly into the exhaust via the dosing valve.
In either case, the dosing valve is mounted in close proximity to the exhaust, and thus operates in a harsh environment where temperatures can reach as high as 600 deg C. Accordingly, the dosing valve must be cooled to prevent decomposition or crystallization of the urea prior to delivery into the exhaust stream, and to maintain the integrity of the valve assembly. To alleviate this problem, prior art expedients have employed water cooling systems for the valve assembly. However, water cooling requires specialized plumbing and additional components that ultimately increase costs and reduce reliability.